If I understand your post correctly I think you said you already have it laced up but have not quite finished tightened the spokes. If so, disregard the following. Otherwise, I should have mentioned the procdure I always use is to first lay the new rim on a flat surface to see that it is flat, and bend it as required to make it as flat as you can (within reason). Next measure its diameter at several places to see that is in round and not oval, again bending as required to reach a degree that you will be satisfied with. Next screw on all the spoke nipples the same amount as accurately as you can judge, going around and round the wheel little at a time. Then proceed with the incremental tightening till they ring, watching the lateral and radial runout as you go.
If you have difficulty getting if trued up to an acceptable level, you can always unscrew all the nipples and start over, being more careful. Just don't be quick to give up. There is nothing that a professional can do that you can't do.